The printing forms in sheet-fed offset printing presses consist of flexible printing plates spanned about a plate cylinder. In order that the printing plate comes to lie in register on the plate cylinder of the printing mechanism. on the edge allocated to the print start of the printing plates there are arranged register cuts in the form of perforations or cuts, which cooperate with corresponding fitting pins or other form parts in the area of the clamp rail. As a result the flexible plate, prior to fastening to the edge assigned to the print start, is aligned both in the lateral and also in the circumferential direction.
The most widespread printing plates for sheet-fed offset printing presses consist of a thin aluminum plate. In such aluminum printing plates it is possible with the register cuts briefly described above, besides register pins, to establish the correct onlay of the printing plate with its cuts on the registering means of the cylinder by an electrical interrogation. For this purpose either the register pins altogether, or parts of these register pins, are arranged electrically insulated with respect to the plate cylinder so that through a printing plate lying correctly on the pins in the zone of the register cuts, the insulation is bridged in the manner of a switch. A printing plate with register cuts arranged on the edge assigned to the print start, which cooperates with electrically interrogatable register pins, is known from DE 42 26 780 C2. From DE 43 38 664 C2 it is known, further, that such electrically interrogatable registering arrangements on the plate cylinder of printing presses make it possible to automate a printing plate change operation. Thus, it is possible to execute the fastening of a new printing plate to be fed in automatically, in dependence on the in-register positioning of the printing plate. Further, it is likewise possible, after opening of the print start clamping bar, to determine whether the used printing plate has been moved out of the clamping gap.
Besides printing plates comprising a thin aluminum plate, in sheet-fed offset printing presses there are also used printing plates in the form of thin plastic foils. The advantage of such plastic foils lies in that with certain film-exposure apparatuses it is possible to expose corresponding coated plastic foils and therewith to structure the ink-receiving from the non ink-receiving plates.
In order to make it possible to fit these plastic foils as printing plates, some are provided with cuts in correspondence with the register system installed on the print start bar. Since the plastic material of such printing foils, however, is not electrically conductive, or is not electrically conductive to the intended degree, an electric register control of the type previously described cannot be used. In a printing press with such an electric register control, this must be switched off when such printing foils are used, or the automated and successively following processes of a printing plate change operation must be monitored and manually controlled by an operator.